Change to figures not saving

Hi All,
I am writing a technical document which includes a number of graphs. I am just about to finish the document so I am doing the last tidy up. However everytime I change anything in the figures (such as font size or the plot area) and then save the document, when I open it up again the changes are not saved. Changes to the document text are saved.

The plots are originally from Excel and the document was originally a docx file, I am saving it in docx.